Immuno-osseous dysplasias (IOD) are a group of disorders characterized by immune deficiency and skeletal dysplasia. Schimke's disease (SIOD) and cartilage hair hypoplasia (CHH) represent the prototypic forms of these disorders, but several other cases of IOD with unknown molecular basis have been described. The immunodeficiency seen in IOD is often severe, and affects predominantly T lymphocytes. Immune dysregulation typical of Omenn syndrome (OS) has been reported in some patients with CHH. We have identified and studied two siblings (a male and a female) with rapidly progressive and fatal IOD. Both infants manifested T cell lymphopenia with markedly reduced number of CD8+ T cells, lack of proliferation to mitogens, hypogammaglobulinemia, increased serum IgE and eosinophilia. The elder sibling had features of OS, with erythroderma and expansion of oligoclonal and activated CD4+ T cells. No mutations were identified in the SMARCAL1 and RMRP genes, responsible for SIOD and CHH, respectively. Whole exome sequencing (WES) revealed that both siblings were homozygous, and both parents heterozygous, for a novel and drastic missense mutation (R339W) affecting a highly conserved residue of Exostosin-like 3 (EXTL3), a member of the exostosin (EXT) family of glycosyltransferases involved in heparan sulfate (HS) biosynthesis. HS are sulfated glycosaminoglycans that decorate cell surface and matrix proteoglycans. HS proteoglycans modulate the activity of morphogens that play a key role in skeletal development and T cell differentiation. Our overall hypothesis is that EXTL3 is critical for thymic development and that EXTL3 mutations in humans cause a novel form of IOD. In support of our hypothesis, our preliminary data demonstrate that EXTL3 is expressed by human thymocytes, thymic epithelial cells (TECs), and peripheral T cells. Moreover, mutation of Extl3 in zebrafish causes defects of limb bud development, and impaired thymic T cell development. To test our hypothesis, and to investigate the cellular basis of the immunodeficiency associated with EXTL3 mutations, we will generate and characterize mouse models with tissue-specific disruption of the Extl3 gene in neural crest mesenchyme cells, TECs, or thymocytes. All of these cell types produce HS and are critically involved in thymus organogenesis and T cell development. In parallel, in order to demonstrate the disease-causing effect of the EXTL3 p.R339W mutation identified in the affected siblings, we will perform disease modeling using patient-derived induced pluripotent stem cells (iPSCs). We will use the CRISPR/Cas9 technology to correct the EXTL3 p.R339W mutation in patient-derived iPSCs. In order to define the cell autonomous effect of the EXTL3 mutation, we will differentiate both mutant and gene-corrected iPSCs to TECs and T cells, and we will study HS biosynthesis under the same experimental conditions. We anticipate that this study will provide novel important insights on the biological role of HSPGs in immune system development and function, and will help define the optimal therapeutic approach to correct the immunodeficiency of this condition.